Apparatus using monochromatic radiation of different wavelengths



May 25, 1954 J. G. ATWOOD 2,679,185

APPARATUS USING MONOCHROMATIC RADIATION OF DIFFERENT WAVELENGTHS FiledAug. 1, 1952 3 Sheets-Sheet 1 INVENTOR ATTORNEY} M y 5, 1954 J. G.ATWOOD 2,679,135

APPARATUS USING MONOCHROMATIC RADIATION OF DIFFERENT WAVELENGTHS A FiledAug. 1, 1952 '3 Sheets-Sheet 2 ATTO RN EY5 May 25, 1954 J. G. ATWOODAPPARATUS usmc MONOCHROMATIC RADIATION OF DIFFERENT WAVELENGTHS 3Sheets-Sheet 3 Filed Aug. 1, 1952 'INVENTOR 72 MM ATTORNEY Patented May25, 1954 STATES rear ()FF'ICE.

APBARATUS; 'J '.r NQCHRD 'AT Q R DIATION OF DIFFERENT WAVELENGTHS.

App ic tion An us, 1 2,. strains 302,181

18 Claims. 1.

Thisinvention relates to apparatus for spectral analysis. photometry,and similar purposes, in which monochromatic radiation at differentwavelengths is employed, an example of the useof such apparatus being aSpectralanal-ysi-sin volving the quantitative determination of thecomposition of a two-component mixture by ascertaining the differencebetween or the ratio of its absorptionsof energy at the selectedwavelengths. More particularly, the invention is concerned" with a novelapparatus of any of the kinds identified, which includes means forsegregating from radiation from a source a pair of monochromatic beamsof different wavelengths.-

The, embodiments of the invention for the several; purposes stated.include the same essentialfeatures butv 'difier among themselves inother respects. Thus, a form of the new apparatus suitable forabsorption'spectrometry includes a source of radiation and a cell forthe sample to be, analyzed, which lies in the path of radiation from thesource, while, in an apparatus involving the invention, which issuitable for Batman spectrometry. the source of the radiation, fromwhich the two beams ar g at d, s the p which is excited by a secondsource. In a damephotometer employing the invention, the source oftheradiation is the sampla'w-hich has been excited by the flame, and anoptical pyrometer, in which the invention is used. receives theradiation from the not object, the temperature of which is to bemeasured. All features of the invention are advantageously utilized in aform of the apparatus for use in absorption spectrometry and.accordingly, a typical absorption spectrometer embodying the inventionwill be illustrated and describedin detail for the purpose of explainingthe principle of the invention.

In a conventional spectrometer, the radiation from a source enters thesystem through an entrance slit, usually passing through the samplefront oi or just behind the slit, The beam is then collimated, dispersedby a prism, and focused upon an exit slit, through which radiation ofthe desired wavelength issues to fall upon a detector, such as aphotocell, thermocouple, etc., respond-.- ing to the radiation reachingit. Such an instrument may be modified by addition of an entrance slit,an exit slit, and a detector to per-- mit its use in analysis of asample by means of radiation of two diiierent wavelengths, but the useof the modified instrument for such analyses is unsatisfactory for anumber of reasons. It is difficult to adjust the instrument; so thatradiation of the desired wavelengths falls upon the respective detectorsand, in addition, errorsin analysis are likely to arise because ofdifferences in response of the detectors, resulting from variations intheir sensitivity.

The present invention is, accordingly, directed to a novel apparatus forspectrornetry, photometry, etc., which overcomes the difiicul-ti'es inprior apparatus utilizing two beams ofinoncchromatic radiation and apair of detectors responding to the energy in; the respeotive beams. Thenew apparatus includesa monochromator receiving radiation from a sourceand provided with an element for dispersing the radiation, and" means,which receive dispersed radiation directly from the dispersingelementanddirect a part of the radiation along a normal opt-icalpath,while deviating the remainder of the radiation from that path. Thedeviated and undev-iated radiation is focused upon an exit slit and thenfalls upon a single detector, and the means referred to are adjustable,so that the undeviated radiation issuing through the exit slit is of oneof the desired wavelengths, while the deviated radiation issuing throughthe slit is of the other of the desired wavelengths. The apparatusisprovided with chopping means operating to cause deviated andundeviated radiation to fall upon the detec-- alternative forms of achopping disc, which may be employed in the instrument;

Fig. 4 is a view in elevation of the prism and deviating means;

Figs. 5, 6, and 7 are diagrammatic plan views showing the operation ofthe deviating means;

Figure 8 is a plan View of partof a modified form of the new instrument;

Fig. 9 is a view in perspective of parts of the instrument shown in Fig.8;; and

Fig. 10 is a diagrammatic view of'apparatus for utilizing the output ofthe detector.

An absorption spectrometer embodying-the invention may be employed indifferent forms for examination of a sample by radiation of 'difierentkinds, and the forms. of the apparatus shown are for use in spectralanalysis by means of infrared 35 radiation. When the new instrument isto be used in analysis by means of other radiation, the source ischanged and other alterations made in the instrument, as will beexplained.

The apparatus shown in Fig. 1 includes a source l emitting infraredradiation including the two wavelengths to be used for analyticalpurposes, and the source may be of any of the kinds commonly used.Radiation from the source falls upon a plane mirror H and is reflectedtherefrom to a spherical mirror [2, which produces an image of thesource at Is. The radiation reflected by the mirror i2 is chopped by achopper it, which includes a rotary disc. The disc may be of variousforms, of which two are shown in Figs. 2 and 3.

The chopping disc i l, shown in Fig. 2, is generally circular in formbut has a peripheral cutout I la extending through an arc ofapproximately 180. semi-circular slot Mb, the cut-out and the slot lyingin opposite halves of the disc. Preferably, the radius of the disc atthe cut-out i i-a is approximately the same as the outer radius of slotMb. When a beam of radiation falls upon the disc adjacent its peripheryas indicated at 13, part of the radiation is passed by the slot Mb andpart is intercepted by the rim of the disc outside the slot. When thedisc is rotated through 180, the part of the radiation previously passedby the slot is intercepted by the disc and the part of the radiationpreviously intercepted by the rim of the disc is passed by theperipheral cut-out Ma.

The chopping disc l5 shown in Fig. 3 is circular in form and it isprovided with an outer semicircular slot a and an inner semi-circularslot Mb. The inner radius or" the outer slot is the same as the outerradius of the inner slot and the slots are angularly off-set so thatthey overlap through an arc of 90. With this arrangement, the disc chopsa beam 3 of radiation falling thereon so that, during rotation of thedisc through 90, radiation is passed by the outer slot only, afterwhich, during the next 90 travel of the disc, radiation is passed byboth slots. Thereafter, for 90 of the travel of the disc, radiation ispassed by the inner slot only, following Which all radiation isintercepted, while the disc moves through 90.

The radiation passed by the chopper i3 falls upon a spherical mirror:16, from which it is reflected to a plane mirror ll lying at adiagonal. The combination of mirrors i6 and l! focuses the radiation onan entrance slit l8, and the radiation passing the slit falls upon aparaboloidal mirror 19. The radiation is collimated by mirror is and thecollimated beam is passed through a prism 26 and dispersed. Thedispersed radiation travels to a Littrow mirror 2!, which returns theradiation through the prism for a second dispersion and also acts as adeviating means.

The mirror 2! is made up of an upper part 2 la and a lower part 21b andthese parts are relatively rotatable upon the same axis, which is normalto the plane of dispersion and, in the instrument illustrated, isvertical. The mirror returns radiation falling thereon to the prism andthe radiation passes through the prism to be dispersed a second time.The radiation leaving the prism after the second dispersion travels tothe mirror l9, which focuses it upon an exit slit 22, the focusedradiation being turned through an angle of 90 on its way to the slit byThe disc also has an approximately 7 are not focused by the mirror uponthe exit i the diagonal mirror 23. The radiation issuing through theslit is caused to fall upon a detector and, in the instrument shown, theradiation travels to a diagonal mirror 24 and thence to an ellipsoidalmirror 25, which focuses the radiation upon detector 26.

As a result of the different angular positions of parts 21a and 2!?) ofthe split Littrow mirror 21, radiation of two different wavelengthsreiieoted from respective parts of the mirror passes through the prismto mirror l9 and is focused by mirror is upon the exit slit, whileradiation or" all other wavelengths reflected by mirror 2| either doesnot reach mirror l9 or else is not focused by mirror i9 upon the exitslit. This efiect of the two parts of the mirror will be apparent fromFigs. 5, 6, and '7, in which the line 2'! represents rays of radiationof many wavelengths traveling from the entrance slit to the paraboloidalmirror H, by which the rays are collimated and sent to the prism. Therays are dispersed by the prism and rays of different wavelengths issuefrom the prism along different paths. Thus, rays of wavelength L1,issuing from the prism along the path 2% (Fig. 5), fall upon the upperpart 2 la of the Littrow mirror and are returned to the prism along path29, while the rays of wavelength L2, issuing from the prism along path30, fall upen the lower part 2!?) of the mirror and are returned to theprism along path 3i. Because of the difference in orientation of the twoparts 2 la, 2 lb of the mirror, the reflected rays L1 and L2, aftertheir second passage through the prism, travel to the paraboloidalmirror 19 in a plane 32 normal to the base of the prism, and are focusedthereby upon the exit slit. Rays of wavelengths other than L1, L2, whichstrike the respective parts 2 la, 2 lb of the Littrow mirror, are notfocused upon the exit slit. Thus, as shown in Fig. 6, a ray may strikethe upper part 21a of the mirror and be returned thereby along a path33, so that, after emergence from the prism, the ray does not reach theparaboloidal mirror l9. Similarly, a ray may strike the lower part 2 lbof the mirror and be returned along a path 34 (Fig. 7) so that it doesnot reach mirror l9. Some rays of wavelengths other than L1, L2 mayemerge from the prism and strike mirror 19, but the angle of incidenceof such rays upon the mirror will be such that these rays The use of thesplit Littrow mirror with the two relatively angularly adjustable partsmakes it possible to cause radiation of two selected wavelengths to befocused upon the exit slit and the wavelengths may be varied as desired.One part of the mirror may be considered to return the dispersedradiation to the prism along a normal optical path for a seconddispersion and the other part of the mirror then performs two functions,namely, returning the radiation to the prism for the second dispersionand deviating the radiation falling thereon relative to that travelingalong the normal optical path. The part of the mirror which effects suchdeviation of the radiation thus performs functions which, in theinstrument illustrated in the copending application, are performedpartly by an ordinary Littrow mirror and partly by separate deviatingmeans.

In the new instrument, the sample cell may be placed at variouspositions and, if the sample is of good optical quality, the position ofthe cell is not critical and, when the sample is gaseous and does notadversely affect the parts of the instruacac rea.

f merit; the. casing or thesin-stnument may some. as the cell;Theicelllfih'has been. shown in Fig. I as disposed in front of theentrance slit [8,. but: it may also be placed beyond the exit slit.22,bietween the entrance slit: and the. parabolcidalmirrorl9, or between.mirror" t9; and; the exit slit.

If. the, sample: is of; bad: optical quality,.thecell.

must be: placed between the source and: the. entrance slit or betweenthe exit. slit and the dentector 2%.;

The. disc: of thezlight chopper t3 -is placed at a: pnintwhere thedeviated: and: undeviated portionss-of'thez radiation are-separated inspace so that it is possible upon rotation-or the chopper to. chop thoseportions of the; beam at a. phase: I

difierence. and: to cause: deviated and: undeviatedl radiation to fall:alternatelyupon the detector. Thus, the light chopper-.may be placedeither close. to the Littrow mirror: 2%. or at-the location.

of: animage; thereof; the: 1atter arrangement: beingpreferred. Forconvenience, the: position of the chopping disc. in. which it. iscapable of'acting to interrupt the deviated and undeviated'portions. ofthe radiation periodically'maybe' termed the efiective position? of the;disc. in relation to the deviating means; In Fig. 1., the chopper'isshown as disposed atthe location. of an image of the Littrow mirror:lying between the image Is of the source and the spherical mirror. it.The location of another image of. the Littrow mirror is indicated by thedotted line Ln lying between the source and the diagonal mirror H.. Whenthe chopper lies close to the Littrow mirror or.

tion of an image of the Littrow mirronthe beams ofradiation, which fallupon the two parts of the- Littrow mirror and may be, considered toconsist, respectively, or" u'ndeviated and; deviated. radiation, areseparated in. space, so that. the. disc can periodically interrupt thebeams out ofiphase.

The signals produced by the detector are amplified in amplifier 3t. andfed to a phase sensitive' rectifier 3?, which also receives the outputof the synchronous generator 38,.which is driven at the same rate as thechopping disc and may have its rotor shaft mounted on the shaft of motor3% driving the chopper. The output of the rectifier may then be led toasuitable recorder 40. The signals produced by the-'- detector vary inamplitude, depending upon the transmission by the sample of theradiation. of the two wavelengths, and, when the chopper includes discM, the output of the rectifier is the difference between the two signalsand may be utilizedito: opera-ts an Optical attenuator located at animage. of. the Littrow mirror and intercepting the devicated orundeviated radiation. The movementof the attenuator resulting in a nulloutput of the rectifier is then a function of the ratio of the energies.in the deviated. and undeviated radiation. Whenthe chopper includes thedisc 15, the phase. sensitive rectifier may bereplaced by suitable.breakers, and filters. to. produce separate signals proportionate totheenergiesinthe deviated and. undeviated. radiation.

It the chopper is beyond. the

. instrument of Fig. 1..

In: the instrument-shown inFi'g; 1,. the radiation; is. passed throughthe prism twice for double dis..- persion. but, when a single dispersionof the. radiation issuificient, the form of'theinstrument illustrated inFigs. 8 and 9 may be employed. In the modified form of the instrument,the. radiation entering the entrance slit. GI from the source (notshown) is directed by the diagonalmirror 42 to the paraboloidal mirrorit, which. collimates the radiation and passes it to the prism 44'. Thedispersed radiation issuing fromthe prism falls upon a flat mirror 4'5made up of an upper part the and a lower part lilbpi'voted to swing onan axis, which is normal to thebase of the prism and may lie at the edgeof the prism. From the mirror #5, the. radiation passes toap-araboloidal mirror 45, which focuses the radiation upon an exit slitll, the radiation stril ing a diagonal mirror t8 and being turnedthereby in its travel from mirror 46 to the slit. 'Be yond. the exitslit, the radiation falls upon a dew tector.

The instrument includes a sample cell 4 9'; which may be placed at thevarious positionsstated in connection with the instrument of Fig.

1. Also, the instrument includes a, light chopper 59 illustrated aslying at an image of the flat mirror 45 ahead or" the entrance slit. Thelightchopper may be placed close to the mirror 45 or at the location ofany of the images thereof in the system.

The two parts its, this of the fiat mirror 4'5- function in mannergenerally similar to the two parts of the Littrow mirror employed intheThe dispersed radiation issuing from the prism strikes the two parts ofmirror 45 and is directed thereby to the parab--- oloidal mirror ddwhich focuses the radiationupon the exit slit. The diiierence in theangular adjustment of the two parts of mirror d5 causes. light of thetwo selected wavelengths only to fall upon the slit and the two beamsthen fall upon the detector alternation. The output of the detector isamplified, rectified, and de-:

, livered to a suitable meter or recorder, asdesired.

The instruments in the forms shown are iii-- tended for use in theinfrared field, and the collimating and focusing elements are mirrors.In the new instruments for use in the visible: field, the collimatingand focusing elements may be front surface mirrors or glass lenses,while, in an instrument for use in the ultraviolet field, thecollimating' and focusing elements may be front surface mirrors orquartz lenses.

In all forms of the new instrument, the deviation of part of the beam iseiiected by a split: mirror, the two parts of which are capable.ofrelative angular adjustment and which is preferably of the Littrow orfiat types. The disper-: sion of'the collimated beam is effected byapri'sm in the instrument shown, but other dispersing elements may beemployed, if desired. The detector used will depend on the kind. ofradiation. employed, as will also the source. In instru-- ments, inwhich a relatively wide entrance slit can be used, the source may beplaced at the focus of the collimating element and the slit dispensedwith. Also, by using a detector hav ing a target of proper width andpositioning the detector at the proper location, the exit slit may bedispensed with. Positioning. the source at the collimator focus is thusfor some purposes the equivalent of using a. source. plus. an entranceslit. Similarly, the use at the proper location.

of a detector having a target of proper width is the equivalent of adetector plus an exit slit. In the instrument, the optical system isreversible and the source and detector may thus be interchanged.

In the use of the new instrument for the analysis of a, sample by meansof radiation of the two selected wavelengths, the sample is introducedinto the sample cell and the source is turned on. The slits and one partof the split mirror are then adjusted until radiation of one of thedesired wavelengths is focused upon the exit slit and falls upon thedetector. The otl part of the split mirror is then angularly adjusteduntil radiation of the other of the two selected wavelengths is focusedupon the exit slit and falls upon the detector. The chopping of theradiation passing to the detector causes the output of the latter to bebroken up in time into two signals which are out of phase, and, byappropriate known means, the signals may be utilized to show thedifference in absorption oi the two wavelengths by the sample. If asec-- ond analysis is to be made by radiation of two differentwavelengths, the slits and one of the parts of the split mirror areadjusted to focus radiation of one of the new wavelengths upon the exitslit, and, thereafter, the radiation of the second new wavelength isfocused upon the slit by angular adjustment of the other part of thesplit mirror. If, in the second analysis, only one new wavelength is tobe employed, the only part of the instrument to be adjusted is one partof the split mirror. In the instrument, the use of a single source and asingle detector eliminates errors rising from deterioration in thesource or a decrease in detector sensitivity caused by aging.

The term wavelength, as used in the foregoing specification and appendedclaims, is intended to refer not to a single wavelength, out to a narrowband of consecutive wavelengths. If desired, the Littrow mirror 2i orthe flat mirror 45 may be split into more than two parts, for example,three, in which event the deviated portion of the radiation may be madeup of two bands separated in the spectrum.

In all forms of the new instrimient, radiation from a source enters amonochromator, which includes an entrance slit or its equivalent, acollimating element, a dispersing element, a focusing element, and anexit slit or its equiva lent. In the instrument of Fig. l, the monochromator is of the double dispersion type and includes a Littrowmirror, and the parabolcidal mirror It is used for both collimating andfocusing. In th instrument of Fig. 8, the monochromator utilizes asingle dispersion and the mirror 4-5 passes the dispersed radiation tothe focusing mirror 56, instead of returning it, as does the Littrowmirror, to the prism for a second dispersion.

The monochromator of the instrument has been modified by incorporatingin it means for performing the function or deviating a portion of thedispersed radiation and, for convenience, the Littrow mirror or mirror55 have been formed of two relatively angular-1y movable parts, whichcan be adjusted, so that the radiation reflected from one part isdeviated in relation to that reflected from the other part. The sameresult can be obtained by the use of reflecting means separate from theLittrow mirror or mirror 45 for effecting the deviation, but theconstructions shown are the simplest and most desirable.

This application is a continuation-in-part of my application Ser. No.233,073 filed June 22, 1951.

I claim:

1. In an apparatus for segregating from radiation from a sourcemonochromatic beams differing in wavelength, the combination of amonochromator receiving radiation from the source and includingdispersing means, and means within the monochromator and separate fromthe dispersing means positioned to receive dispersed radiation directlyfrom said dispersing means, said receiving means directing part of thedispersed radiation along one path and another part of the dispersedradiation along another path, a single detector, means for causingmonochromatic beams of radiation difiering in wavelength and leaving thedirecting means along the respective paths to fall upon the detector,and means disposed at an effective position relative to the directingmeans for periodically interrupting at a phase difierence the radiationtransmitted to the detector along the two paths.

2. In an apparatus for segregating from radiation from a sourcemonochromatic beams differing in wavelength, the combination of amonochromatcr receiving radiation from the source and includingdispersing means, and reflecting means within the monochromator separatefrom the dispersing means and positioned to act on the dispersedradiation and to reflect part of the radiation along one path andanother part of the dispersed radiation along another path, a singledetector, means for causing monochromatic beams of radiation diiferingin wavelengths and leaving the reflecting means along the respectivepaths to fall upon the detector, and means disposed at an effectiveposition relative to the reflecting means for periodically interruptingat a phase difference the radiation transmitted to the detector alongthe two paths.

3. In an apparatus for segregating from radiation from a sourcemonochromatic beams differing in wavelength, the combination of meansfor producing a collimated beam of radiation from the source, means fordispersing the beam, and reflecting means separate from the dispersingmeans positioned to have the dispersed beam falls thereon and to reflectpart of the dispersed radiation along one path and another part of theradiation along another path, a single detector, means for causingmonochromatic beams of radiation diifering in wavelength and leaving thereflecting means along the respective paths to fall upon the detector,and means disposed at an effective position relative to the reflectingmeans for periodically interrupting at a phase difference the radiationtransmitted to the detector along the two paths.

4. In an apparatus for segregating from radiation from a sourcemonochromatic beams differing in wavelength, the combination of meansfor producing a collimated beam of radiation from the source, means fordispersing the beam, reflecting means separate from the dispersingmeans, upon which the dispersed beam falls and which directs part of thedispersed radiation along one path and another part along another pathat an angle to the first, a single detector, means for causingmonochromatic beams of the radiation diiiering in wavelength and leavingthe reflecting means along the respective paths to fall upon thedetector, and means disposed at an effective position relative to thereflecting means for periodically interrupting at -a phase differencethe radiation transmitted 'to the. detector along'the two paths.

5. "In an apparatus for segregating-from radiation from a sourcemonochromatic beams differing in wavelength, the combination of meansfor-producing a collimated beam of radiation from the source, means fordispersing the beam to form a spectrum, a mirror, upon which thedispersed beam falls and which comprises two parts angularly ofisetrelative to'each otherabout an axis normal to the plane of dispersion, asingle detector, means for causing monochromatic beams of radiationdiffering in wavelength and reflected by the respective parts of themirror to fall upon the detector, and means disposed at an effectiveposition relative :to the 'smirror for periodically interrupting at aphase difference the radiation transmitted to the detector along the twopaths.

6. "In an apparatus for segregating from radiation from a sourcemonochromatic beams differing in wavelength, the combination of meansfor producing a collimated beam of radiation from the source, a prismfor dispersing the collimated beam, a mirror, upon which the dispersedbeam falls and which comprises two parts having faces normal to theplane of the base of the prism and angularly ofiset relative to eachother about an axis normal to said plane, a single detector, means forcausing monochromatic beams of radiation differing in wavelength andreflected by the respective parts of the mirror to fall upon thedetector, and means disposed at an effective position relative to themirror for periodically interrupting at a phase difference the radiationtransmitted to the detector along the two paths.

'7. In an apparatus for segregating from radiation from a sourcemonochromatic beams difiering in wavelength, the combination of meansfor producing a collimated beam of radiation from the source, means fordispersing the beam to form a spectrum, a mirror, upon which thedispersed beam falls and which comprises two parts angularly ofisetrelative to each other about an axis normal to the plane of dispersion,a single detector, means for causing monochromatic beams of radiationdiffering in wavelength and reflected by the respective parts of themirror to fall upon the detector, and radiation chopping means disposedin an efiective position relative to the mirror to cause radiationtransmitted to the detector along the two paths to be periodicallyinterrupted at a phase difference.

8. In an apparatus for segregating from radiation from a sourcemonochromatic beams differing in wavelength, the combination of meansfor producing a collimated beam of radiation from the source, means fordispersing the beam to form a spectrum, a mirror, upon which thedispersed beam falls and which comprises two parts angularly oiTsetrelative to each other about an axis normal to the plane of dispersion,a single detector, means for causing monochromatic beams of radiationdiffering in wavelength and trance slit, means for dispersing radiationfrom the source admitted by the entrance slit, a mirror, upon which thedispersed beam falls and which comprises two parts having faces normalto the plane of dispersion and angularly displaced relative to eachother about an axis normal to said plane, a single exit slit, means forreceiving radiation from the two parts of the mirror and focusing theradiation upon the exit slit, a single detector receiving radiationissuing through the exit slit, and radiation chopping means disposed inan eifective position relative to the mirror, in which said choppingmeans acts to cause radiation reflected from respective parts of themirror to strike the detector'intermittently and at a phase difference.

10. In an apparatus for segregating from radiation from a sourcemonochromatic beamsdi-ffering in wavelength, the combination ofwanwentrance slit, a single exit slit, a dispersing element between theslits, an optical component receiving radiation through the entranceslit and collimating the radiation and passing the collimated beam tothe dispersing element for dispersion, reflecting means separate fromthe dispersing element receiving the dispersed beam from the dispersingelement and returning part of the radiation along one path and anotherpart of the radiation along another path, both reflected parts of theradiation traveling to the dispersing element for a second dispersionand the twice-dispersed radiation falling upon the component and beingfocused thereby upon the exit slit, the radiation issuing through theexit slit being made up of monochromatic radiation of two wavelengths, asingle detector, upon which said monochromatic radiation falls, andradiation chopping means disposed at an effective position relative tothe reflecting means to cause radiation of said two wavelengths to fallupon the detector intermittently and at a phase differonce.

11. An apparatus suitable for use in absorption spectrometry, whichcomprises a source of radiation, means for producing a collimated beamof radiation from the source, an element for dispersing the collimatedbeam, a mirror, upon which the dispersed radiation falls and whichcomprises two parts angularly offset relative to each other about anaxis normal to the plane of dispersion, a single exit slit, meansreceiving radiation reflected by the respective parts of the mirror andfocusing said radiation upon the exit slit, a single detector, uponwhich radiation issuing through the exit slit falls, and radiationchopping means in an effective position relative to the mirror to causeradiation reflected by the two parts of the mirror to fall upon thedetector intermittently at a phase difference.

12. An apparatus suitable for use in absorption spectrometry, whichcomprises a source of radiation, an optical component receivingradiation from the source and producing a collimated beam thereof, anelement for dispersing the collimated beam, a mirror, upon which thedispersed radiation falls and which comprises two parts angularly oiisetrelative to each other about an axis normal to the plane of dispersion,the mirror returning the dispersed radiation to the elea sample cell inthe path of the radiation traveling from the source to the detector, andradiation chopping means disposed at an efiective position relative tothe mirror and acting to cause radiation reflected by the two parts ofthe mirror to fall upon the detector intermittently at a phasedifference.

13. An apparatus suitable for use in absorption spectrometry, whichcomprises a source of radiation, an optical component receivingradiation from the source and producing a collimated beam thereof, anelement for dispersing the collimated beam, a mirror, upon which thedispersed radiation falls and which comprises two parts angularly offsetrelative to each other about an axis normal to the plane of dispersion,an exit slit, an optical component focusing radiation reflected from themirror upon the exit slit, a single detector, upon which radiationissuing through the exit slit falls, a sample cell in the path ofradiation traveling from the source to the detector, and radiationchopping means disposed at an effective position relative to the mirrorand acting to cause radiation reflected by the two parts of the mirrorto fall upon the detector intermittently at a phase difference.

References Cited in the file of this patent UNITED STATES PATENTS

